This invention relates to an electric heater and, specifically of a type for producing a relatively high heat concentration in a small volume useful for the quick sterilization of probes and the like. For example, such electric heaters may be found in medical laboratories where probes and the like are used for handling test specimens and must be sterilized periodically.
By way of background, electric heaters of this general type comprise a core made of a ceramic material that is first machined to the desired shape, and then fired or cured. The core has a shank portion of constant cross section and an enlarged neck portion, generally funnel shaped, for ease in inserting a probe or the like therein for sterilization. A wire type heating element is wound around a portion of the neck at the location of external threads formed thereon, and also along a substantial portion of the shank. A cement having high temperature and good bonding characteristics is coated over the heating element, and an outer covering of felt also having high temperature characteristics is wrapped around the cement. Electrical leads are provided for applying electrical power to the heating element.
The procedure in making the prior art heaters was to first machine the core to the proper shape and dimensions, and then fire or cure it. Thereafter, the heating wire was wound onto the core, a coating of cement applied over the wire which bonded the wire to the core, and the felt covering than wrapped over the cement, such that the cement bonded the felt covering, wire and core together.
While these prior art heaters were operable to an extent, they were possessed with certain problems. One such problem was that the heating element, and thus the heater, failed sooner than was acceptable. This is believed to have been due to expansion and contraction of the core upon energizing and deenergizing the heating element which, because the heating element was bonded to the core, in turn caused expansion and contraction of the heating element and finally its failure.
Another problem with the prior art heaters was that the core had relatively high leakage characteristics so that the user would often receive an electrical shock if the probe he was sterilizing and holding in his hand touched the inner surface of the core. With heaters of this type that operated at the higher voltages, such as 240 volts, the shock was more severe and quite dangerous.
This invention, including the heater and method of making same, has overcome these problems to provide a heater that has exceptionally long life, exceptionally low leakage characteristics, and is of generally higher quality than the electric heaters of this type found in the prior art.